CAL 

 BOX 



SOUND-LEVEL 

 METER 



BATTERY-OPERATED 



PORTABLE TAPE 



RECORDER 



12 INCHES 



HYDROPHONE-PREAMPLIFIER 



Figure 2. The Sound-Measuring System shown with a portable tape recorder. 



Hydrophone and Preamplifier 



The hydrophone -preamplifier assemblies (M-H90-A) 

 were manufactured by Wilcoxon Research Co. , Bethesda, 

 Md. , according to the specifications which are included in 

 Appendix B. The crystal material used in PZT4 and the 

 construction of the hydrophone is such that the acceleration 

 sensitivity is reduced to a minimum. The acceleration 

 sensitivity is given as less than +60 dB//ubar/g. The source 

 capacitance, sensitivity, and input impedance can be speci- 

 fied to obtain a hydrophone with a low input noise level and 

 sufficient sensitivity to measure the lowest ambient noise 

 levels in the frequency range of interest. The crystal was 

 specified with a source capacitance of not less than 1000 pF 

 with a sensitivity of not less than -90 dBV re 1 dyn-cm" 2 . 

 The input impedance of the preamplifier was specified to be 



